KABUL, Afghanistan – A suicide attacker with a bomb in his turban posed as a Taliban peace envoy and assassinated a former Afghan president who for the past year headed a government council seeking a political settlement with the insurgents.

The attack Tuesday, carried out in former President Burhanuddin Rabbani’s Kabul home, dealt a harsh blow to attempts to end a decade of war. The killing of Rabbani, an ethnic Tajik and one of the wise old men of Afghan politics, will blunt efforts to keep in check the regional and ethnic rivalries that help feed the insurgency.

President Hamid Karzai cut short a visit to the United Nations and called on Afghans to remain unified in the face of Rabbani’s “martyrdom.” An emergency Cabinet meeting was called for today.

The attack came days after a daytime assault by insurgents on the U.S. Embassy and NATO headquarters that deepened a sense of insecurity in the capital.

NATO said in a statement that two suicide bombers were involved in the attack on Rabbani, both of them men who had feigned a desire to reconcile with the government. It was unclear if a second bomber was able to detonate his explosives.

Afghan officials, however, insisted there was only one attacker. Four men were wounded, including a key presidential adviser, said Mohammad Zahir, the head of criminal investigations for the Kabul police. Initial reports had four bodyguards killed, but Zahir said those reports were incorrect.

Close friends of Rabbani said that the former president, who was about 70, returned from a trip to Iran to meet with a man who had been described as a high-ranking Taliban contact. The visitor, a young man, was shown into the house by two of Rabbani’s associates at the Afghan High Peace Council, who insisted that he did not need to be fully searched, said a friend who spoke anonymously because he was not a spokesman.

When Rabbani appeared, the man shook the former president’s hand and bowed as a sign of respect, said Fazel Karim Aimaq, a former lawmaker from Kunduz province and friend of Rabbani.

“Then his turban exploded,” Aimaq said. The blast broke windows in Rabbani’s home and shook nearby houses.

As the leader of the anti-Taliban Northern Alliance, Rabbani sought a political deal with the Taliban – with Washington’s blessing – and he will be hard to replace soon. His death could unleash a well of resentment among some senior Northern Alliance members, who accuse Karzai of colluding with the Taliban.

Afghanistan’s ethnic minorities already have begun to rearm in the face of negotiations with the Taliban, who are mostly ethnic Pashtuns, as is Karzai. Rabbani’s killing is likely to accelerate that process and lay the foundation for a possible civil war once U.S. combat troops leave the country or take on support roles by the end of 2014.

In Kandahar province, the birthplace of the Taliban movement, officials worried that the assassination would dampen peace efforts.

“It is a great loss not only to the peace program, but for this nation as well,” said Atta Mohammad, who heads up reconciliation efforts in the province. “This bombing will have a big impact on the peace program for some time.”

President Obama said the killing will not deter the United States and Afghanistan from helping that country’s people live freely. He said Rabbani’s death is tragic because he was a man who cared deeply about Afghanistan. Obama commented at the start of a meeting in New York with Karzai.